1. Field of the Invention
This invention relates to control actuators for aircraft and, more particularly, to a combined series, trim, feel and boost actuator for an aircraft control member.
2. Description of the Prior Art
Typically, in the retrofit aircraft control system market, particularly control systems for helicopters, costly modifications to the existing control systems are required to install the necessary stabilization augmentation and automatic flight control system actuators. Such actuators are of two types: series and parallel, both of which have their advantages and disadvantages. Moreover, in order to provide a complete automatic control system, the following are usually required: a plurality of actuators, an artificial feel spring, a magnetic brake, a trim motor, hydraulic or electric boost servos, control position transducers and control stops. As the above items are typically required for each axis of control, the retrofit becomes extremely difficult and time consuming adding not only to the system's weight but also to the total cost thereof. Even in original helicopter installations, the mounting of separate series and trim actuators and their associated components is difficult and costly.
As mentioned above, actuators may be of two types: parallel and series. The parallel actuator is coupled or bolted directly to the vehicle and thus is subjected to minimum vibration resonances. Furthermore, in a parallel actuator system, the operation is a "hands off" mode wherein the parallel actuator moves both the control surface and the control stick. In a series mode of actuator operation, however, the actuator is not normally directly coupled to the aircraft but, rather, is coupled between the pilot's stick and the control surface, that is, floating or ungrounded. The series actuator, moreover, does not move the stick and has a limited control authority to drive the control surface. Furthermore, as the series actuator is not usually mounted to the vehicle, it is subject to vibration resonances of the control rod in which it is normally installed.
Accordingly, it is desirable to provide an actuator which provides in a single apparatus similar to a conventional parallel actuator, the functions of series actuation, trim actuation, artificial feel, control position sensing, control limits and, if needed or desired, force boost.